The chemical behavior of C.sub.60 and C.sub.70 (fullerenes) is similar to those of linear quasi-one-dimensional conducting polymers, such as polyacetylene, in several important ways. This similarity is due, in part, to an extended network of conjugated polarizable delocalized .pi.-electrons which are distributed over the spherical fullerene surface. As is the case with many conjugated polymers, the electrical conductivity of C.sub.60 and C.sub.70 can be tailored to span the insulating, semiconducting, and metallic regimes by controlled n-type doping. Fullerenes and derivatives thereof also possess unique nonlinear optical properties such as optical limiting. The optical properties of these materials are similarly due to the highly polarizable conjugated .pi.-electrons. However, obstacles to using fullerenes for nonlinear optical and semiconductor applications include their environmental instability under ambient conditions, and the fact that optical quality components are not easily manufactured from the neat materials. Since environmental factors are known to cause chemical modification of the fullerenes that effect these important electronic and optical properties, it becomes important to protect the C.sub.60 and derivatives thereof, in order to preserve their unique electrooptical properties in device applications.
Composites of fullerenes in optical glass matrices would be of interest for applications including optical limiting and nonlinear optical waveguides. Several authors have reported optical limiting in C.sub.60 organic solutions. This limiting effect is due to the very efficient intersystem crossing; that is, there is a significant triplet-triplet absorption in the visible spectral region, following photoexcitation into a singlet excited state. As the incident light intensity increases, the transmitted intensity is limited, approaching a constant value (&lt;15 mJ/cm.sup.2) at high laser input intensities. Optical limiting would be useful for eye protection from pulsed laser light if fullerenes were to be protected in an optically transparent isolation matrix. In U.S. Pat. No. 5,172,278 for "Buckminsterfullerenes For Optical Limiters," which was issued to Lee W. Tutt on Dec. 15, 1992, it is suggested that C.sub.60 compounds may be embedded in a host matrix of a substantially transparent material, such as silicon oxide, silicon nitride, silicon oxynitride, and transparent plastics, such as polycarbonate, polymethyl methacrylate, paralene, styrene, and the like, in order to make composite films. However, no description was provided as to how such embedding might be achieved, and there was no indication that useful bulk samples might be synthesized.
Large values have been reported for the nonresonant third-order optical susceptibility .chi..sup.(3) for C.sub.60. This value is expected to increase, as a function of increasing electron density in n-doped C.sub.60, as well as in higher order fullerenes (and their doped derivatives). This physical property permits the control of light using light for optical switching applications.
A frequently encountered synthetic procedure in sol-gel chemistry involves the polymerization of a metal alkoxide followed by densification to form a fully dense glass. This procedure is known to lead to high optical quality glasses and may be used to include optically active organic species at reduced temperatures compared to conventional melt processing. Formation of glasses by this procedure is a four-step process consisting of solution formation, gelation, drying, and densification. Solutions are formed by mixing metal alkoxides together with a sol-compatible solvent and water, in the presence of an acid or base catalyst. The hydrolysis reaction of a silicon alkoxide produces silanol groups (Si--OH), which subsequently polymerize to form siloxane (Si--O--Si) rings and chains. The following reaction illustrates the acid-catalyzed hydrolysis of tetraethylorthosilicate (TEOS) to form a silica gel: EQU n Si(OC.sub.2 H.sub.5).sub.4 +4n H.sub.2 O.fwdarw.n Si(OH).sub.4 +4n C.sub.2 H.sub.5 OH EQU n Si(OH).sub.4 .fwdarw.(SiO.sub.2).sub.n +2n H.sub.2 O
The concentration of water in the sol plays an important role for this synthetic route leading to porous gels. It is apparent, from balancing these equations, that the minimum amount of water necessary to hydrolyze the alkoxide reaction is 2 moles of water per mole of Si. A minimal amount of water in the sol (2.5 mole ratio of Si to H.sub.2 O) is desired for gel synthesis.
Gelation occurs as the condensation polymerization reaction progresses. Solutions are typically covered with a semi-permeable barrier that allows reaction products to slowly evaporate with time. Homogenous dispersions of primary silica particles having approximately 1-2 nm diameter, coalesce during the early stage of the hydrolysis reaction. Secondary particles condense into 5-10 nm diameter aggregates. The intrinsic viscosity of the solution increases until the solution no longer flows. Gelation occurs at this irreversible sol-gel transition point. At this stage small pinholes are added to the barrier to accelerate the removal of the remaining liquid phase. During the drying period, cross-linking between the secondary particles continues to occur and the bulk gel shrinks (syneresis). The air-dried gel has a porous structure with a pore-size distribution in the range of 20-200 .ANG.. The porosity of the final dried gel can be controlled by altering processing parameters such as hydrolysis time, pH, water content, and heat treatment. The gel may then be thermally annealed (700.degree. to 1200.degree. C.) to form a fully dense glass. Fullerenes are thermally stable in an inert atmosphere at least to about 700 .degree. C.
The feasibility of synthesizing composite gels by inclusion of optically or electrically active organic guest species has been demonstrated in a number of systems over the past few years. Tunable solid-state dye lasers have been successfully fabricated using sol-gel methodology. Conjugated polymers such as polyparaphenylenevinylene, polypyrrole, and polyaniline have been incorporated into gels via the sol-gel technique. Such composites provide the benefit of a low optical loss inorganic oxide glass along with the desired optical nonlinearities or electrical properties of the guest species. With appropriate processing protocols, composite gels can be formed into monoliths, fibers, or films for the exploitation of desired physical properties of the guest species. However, one drawback with composite glasses reported previously has been that the guest dopants were all organic, and hence, decomposed at temperatures required to form the fully dense glass.
In "The Encapsulation of Organic Molecules and Enzymes in Sol-Gel Glasses: Novel Photoactive, Optical, Sensing, and Bioactive Materials. A Review," by D. Avnir et al., NTIS NO. AD-A244 154/1/HDM, Report No. R/D-5548-MS-01 (1992), and in Chapter 65, Polyaniline-ORMOSIL Nanocomposites, by S. J. Kramer et al., of Chemical Processing of Advanced Materials, 1992 John Wiley and Sons, Inc., Larry L. Hench and Jon K. West, editors, the authors describe the incorporation of organic and bioorganic molecules and conjugated polymers, like polyaniline, respectively, into rigid, silica-based matrices using the sol-gel process.
In "Preparation of C.sub.70 -doped Solid Silica Gel via Sol-Gel Process," by Sheng Dai et al., J. Am. Ceram. Soc. 79, 2865 (1992), the authors describe the preparation of transparent oxide glasses containing C.sub.70 by hydrolysis and condensation of alkoxide precursors. The C.sub.70 becomes entrapped in the growing covalent gel network rather than being chemically bound to the inorganic matrix. It is well known that C.sub.60 has much poorer solubility in solvents than C.sub.70, so it would not be expected that glasses involving C.sub.60 and derivatives thereof could be fabricated. In fact, the authors reported that "Application of the same process to C.sub.60 resulted only in a purple silica powder gel.", that is, the method was not effective for C.sub.60.
Accordingly, it is an object of the present invention to microscopically disperse C.sub.60 fullerenes and derivatives thereof within a glass gel.
Another object of the invention is to microscopically disperse C.sub.60 fullerenes and derivatives thereof within a fully dense glass.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.